The invention concerns grain separation in a grain harvester and more particularly a foraminous separating grate or concave of the type which at least partially wraps a separator rotor or threshing cylinder, and cooperates with such rotating element to define a separating and/or threshing zone where crop material is processed and divided and grain and smaller fractions of the material pass through the foramina, typically to a cleaning shoe.
The individual openings (foramina) of the conventional separating grate or bar-type threshing concave are of slot or cell form, usually elongated in the circumferential direction. In one form, especially common as a threshing concave, the openings are defined by spaced apart axially extending bars through which circumferentially extending wires or rods are threaded to create a series of rectangular slots or cells. In another form, common in axial form rotary separators, the foramina are oval or rectangular slots punched in a sheet metal portion of a cylindrical separator casing surrounding a rotor.
In the operation of threshing and separating devices having concaves or grates with rectangular or oval slots of limited circumferential extent, the rotating element [threshing cylinder or separating rotor] impels or drags crop materials circumferentially over the foraminous surface and the rotationally downstream end of a slot acts as a barrier or dead end with respect to flow of some of the material. This general configuration of grate or concave is structurally convenient and the interruptions in the slots or openings contribute to the separating or threshing function by providing a series of spaced working surfaces or impedances. However, in some operating conditions, particularly when crop moisture is high, the impedance becomes too great and material begins to hairpin or mat over the grate structure [and the rotor is unable to keep the grate clear]. In corn harvesting for example, stalks, leaves or silks, hairpin or paste over the slot walls [or rods in a conventional bar concave], and ends until the openings are plugged. In dry conditions crop material may lay as a mat on top of the concave rods and retard flow of grain through the grate.
In separating grates and threshing concaves, grid or cell opening geometry is a compromise between achieving desired separating efficiency and avoiding choking or plugging. The problem of plugging cannot be solved by making the grate apertures larger. Desired separation level would be lost and too much coarse material would go through to the cleaning shoe.
Thompson, in U.S. Pat. No. 1,191,953, discloses a threshing and separating concave for use with a spike tooth threshing cylinder in which adjustably reclining peg teeth emerge from slots between circumferential grate bars. The grate bars are supported and spaced by cross bar members whose inverted triangular cross-section, Thompson claims, helps prevent their collecting material. He claims that his grate design is virtually unpluggable, but the basic form is still a rectangular cell with potential for collecting material by wrapping or hairpinning especially at the closed end of the cells defined by the cross bars.
Landis, in U.S. Pat. No. 959,561 claims to have reduced the possibility of blockage in a purely separating grate by creating a two surface arrangement in which a radially inner surface of substantially continuous circumferential vanes supports straw away from contact with any transverse member. But of course in this arrangement the functional contribution of transverse members in the threshing or separating surface is missing. And, even though the transverse members are offset considerably below (radially outwards) the circumferential vanes it is still possible for them to collect material and develop a blockage.